Interior permanent magnet synchronous machine

ABSTRACT

A rotating A.C. synchronous interior permanent magnet machine with concentrated windings is constructed such that each pole on the secondary-side (rotor) is provided with at least one interior axial slot ( 1 ), with dimensions h 1  ( 6 ), h 2  ( 7 ) and d ( 9 ), with h 1  ( 6 ) being the radial material thickness of the secondary side between the air-gap surface of a slot ( 1 ) and the secondary side air-grap surface, h 2  ( 7 ) being the radial thickness of the secondary side between the inner surface of the secondary side and the inner side surface of the at least one slot and d ( 9 )being the thickness of the secondary side, whereby the ratio (formula I) lies in the range 0.05-0.35.

The present invention relates to an interior permanent magnet,synchronous machine or IPMSM, with concentrated windings, capable offield weakening operation.

Interior magnet motors have been reported in the literature, and theirtorque generation characteristics are well understood, an example ofthis kind of machine can be found in EP 0 823 771 and a technicaldescription of the method of torque generation in Chapter 6 of“Brushless Permanent-Magnet and Reluctance Motor Drives” by T. J. MillerISBN 0-19-859369-4.

The general advantage of this kind of motor is its ability to generateboth a synchronous torque and a reluctance torque which, given asuitable power amplifier, can be independently controlled allowing it torun at speeds far higher than the natural synchronous speed of themotor, and thus to deliver torque and power over a very wide speedrange.

A further advantage is the efficiency of the winding method which allowsmaximum fill-factor while elimination the end-winding losses of adistributed winding construction. The wide speed range capability of aIPMSM is a widely used feature of the standard induction motor, afeature which is not available to any useful extent with a standardsynchronous permanent magnet motor. Synchronous motors are however moredynamic than equivalently sized induction motors, i.e. in rotationalmotors the ratio of torque to rotor inertia can be greater for thesynchronous machine, and they would be useful in machine toolapplications such as spindle drives were it not for the maximum speedlimitation. The IPMSM in conjunction with a suitable active power sourceovercomes this limitation.

The problem with currently available motors are that their operatingspeed range is too narrow, and their secondary side (rotor) constructiontends to complicated by a requirement for special cut-outs or unusualforms or shapes in order to generate the required difference between thedirect and quadrature axis inductance. It is unusual to find an IPMSMwhich has a speed range of much more than 5:1 i.e. where the ratio ofthe maximum allowable speed to base speed is greater than 5:1. As thebase speed of a synchronous motor tends to be around 3000 rpm, thismakes it difficult for such motors to be used in applications wherespeeds of more than about 15,000 rpm are required. Many spindle drivesrun at 25,000 rpm or more which requires a field-weakening range ofaround 9:1 which is not possible with any currently available design.

It is therefore an aim of the current invention to overcome the speedrange limitations and manufacturing complexities of current designs ofIPMSM's, this aim is achieved as described in claim 1.

A rotating A.C. synchronous interior permanent magnet machine withconcentrated windings is constructed such that each pole on thesecondary-side is provided with at least one interior axial slot (1),with dimensions h1, h2 and d, with hi being the radial materialthickness of the secondary side between the air-gap surface of a slotand the secondary side surface, h2 being the radial material thicknessof the secondary side between the inner surface and the inner-lying wallof the at least one slot and d being the thickness of the secondaryside, whereby the ratio h1/d lies in the range 0.05-0.35

Furthermore the range of values of h2/d is in the range 0.4-0.8. Theranges for the ratios of h1/d and h2/d in this design allow thepermanent magnets to be located very close to the surface of thesecondary side and also allow the total thickness of the secondary sideto be reduced, and/or thicker magnet material to be used. Thus in acylindrical motor, for a given motor diameter, the motor shaft can havea larger diameter which provides a greater shaft stiffness and thusbetter motor performance. The dimensions of the iron above and below thepermanent magnets also define the magnetic flux paths through the motorand the levels of saturation and thus the inductance or reluctance ofthese paths. The combination of the geometry of the iron and that of themagnets are the key to increasing the speed range of the motor.

A range of possible values for the ratios h1/d and h2/d exist withinwhich the motor has enhanced field weakening characteristics, howeverdesign and experimentation has shown that a range of 0.18-0.23 for h1/dand a range of 0.55-0.63 for h2/d offer optimum mechanical stability andelectromagnetic performance.

The clear advantage of reduced cogging torque has been achieved in thisdesign by selecting the tooth geometry such that the width of theair-gap around the circumference of the machine is variable. Thus thedetent-torques generated by the interaction of each tooth and magneticpole are not in phase and tend to cancel when summed around the machine.

An optimal solution is the use of a simple rectangular tooth profile inthe primary side laminations resulting in a substantially cuboid toothand thus a flat air-gap surface which creates a variable airgap with thecurved surface of the rotor of the machine.

Advantageous, in the construction of a rotating machine, is the use oftrapezoidal secondary side slots which mean that the section of materialformed between two such adjacent slots has substantially parallel sides.This minimises the volume of iron material in the section and reducesthe hysteresis losses in the machine.

Crucial to the application of the invention to rotating machines is thefact that the concentrated winding construction of the machine generatesa rotating flux which is rich in harmonics, This design is optimised foroperation with the first harmonic which is at double the fundamentalfrequency of the air-gap flux, has a shorter wavelength and can beforced to flow in paths through the iron that the fundamental frequencycan not

A further beneficial aspect of the invention can be seen in itsapplication to a linear machine whereby the useful ratios of h1/d andh2/d have been found to be 0.05-0.35 and 0.4-0.8 respectively.

Further optimisation of the invention have shown that for linearmachines ratios of h1/d and h2/d of 0.18-0.23 and 0.55-0.63 provideoptimal mechanical and electromagnetic characteristics.

The benefits of applying a concentrated winding construction to a linearrealisation of the invention can be seen in that there is a moving fluxfield created by the interaction of the phase currents and the windingswhich is rich in harmonics, the design being specifically andbeneficially optimised for the first harmonic.

A further benefit of the invention is the reduction of cogging torque ina linear motor implementation achieved by designing the tooth geometrysuch that the air-gap between the tooth face and the secondary sidevaries across the width of the tooth thus causing the detent-torques dueto the interaction of the teeth with the permanent magnetic field of themotor from summing constructively along the length of the motor.

In a further beneficial form of the invention, a linear motor can beconstructed such that the cogging torque is minimised by one or all ofthe following:

a) Use of variable thickness magnets

b) Use of magnets with variable flux density across their width

Both of these result in variable flux density in the motor air-gap andminimised detent or cogging torque.

Yet a further advantage of the construction is that it allows a motor tobe designed with a smooth secondary side surface. This substantiallyreduces the costs of manufacturing the machine.

A further benefit of the invention is the design of the slots thatallows the use of 1 magnet per slot. Each the magnet can be simplypushed into each secondary side slot thus significantly simplifying theconstruction of the motor and reducing its manufacturing costs. This isachieved by choosing the width of the magnet to be somewhat less thatthe wavelength of the relevant frequency of the flux field, thusensuring that there is no circulating flux within the body of a magnetand thus reduced losses.

A further beneficial aspect of the invention is the design of the slotsand their separation which ensures that the section of material betweenslots within one secondary side pole is as small as possible andpreferably parallel sided regardless of motor type ensuring reduced ironlosses in the machine.

A further advantage of the invention can be seen in the minimisation ofthe volume of material in the sections of iron between the slots withinone secondary side pole with the result that the iron is permanently inmagnetic saturation even at low flux levels thus further minimising theiron losses in the machine.

In a yet further advantage of the invention, the proximity of the slotsto the surface of the secondary side allows the permanent magnetmaterial to be inserted into the slots in a non-magnetised state whichis far easier than handling magnetised material, and for the permanentmagnet material to be magnetised after construction. This is possiblebecause the very short iron paths through which the magnetising fluxmust flow allow the process to be accurately controlled.

The invention can be more clearly understood by reference to thefollowing diagrams:

FIG. 1 shows an axial view of a rotating machine according to theinvention

FIG. 2 shows an expanded axial view of a single slot of the rotationalmachine of FIG. 1

FIG. 3 shows a linear motor according to the invention with curved toothfaces

FIG. 4 shows a linear motor according to the invention with shapedmagnets

FIG. 5 shows a detailed view of the slots of the linear motor of FIG. 3

FIG. 6 shows a detailed view of the slots of the linear motor of FIG. 4

FIG. 1 shows an axial view of a rotational machine according to theteachings of the invention with a number of individual teeth 3distributed symmetrically around the periphery of the primary side 2.Each tooth 3 having one winding 13 and each tooth 3 having a flat lowersurface 23 which creates the required variable width air gap 10 forminimisation of the detent or cogging torque in the machine. Thesecondary side 4 is a laminated iron construction with the magnet slots1 being stamped or otherwise formed in each individual laminations, thelaminations being stacked together to form the secondary side 4 of themotor. In FIG. 2 it can be readily seen that each slot 1 issubstantially trapezoidal in form, the centre line of each slot 24 lyingon a radius of the machine 28 with the result that the section ofmaterial 15, 16 between slots is parallel sided and rectangular inshape. Rectangular slots 1 would result in larger trapezoidal sectionsof material 15 and increased iron losses as the sections of iron 15would tend to remain in the linear operating range of their B-H curve upto higher flux levels, thus adding to the hysteresis and iron losses inthe secondary side. These sections 15 are necessary to stabilise thesecondary side mechanically for high-speed operation

The sections of iron 16 between secondary side poles 20 is wider thanthat 15 between slots within a pole 20, and is designed such that thesection of material 16 is driven into saturation during normal operationthus increasing the inductance of the path along the q axis of themachine without the need to remove iron to create an air pocket in theflux path in order to increase the inductance. The advantage of thisapproach being that the material above and below the section 16 is notdriven into saturation and is still capable of carrying the requiredmagnetic flux generated by the permanent magnets and by the currents inthe coils during normal operation i.e. the iron provides a lowerreluctance path for the operating flux which would otherwise be forcedto flow across the much higher reluctance air-gap of the machinegenerating a large voltage drop in the air-gap. The magnetic fluxgenerated by the permanent magnets also flows through a shorter returnpath through the air gap further reducing the voltage drop across theair-gap reactance.

A further advantage of not requiring cut-outs or air pockets in the ironis that less tooling is required for production of the secondary side 4laminations which significantly reduces the overall tooling costs andthus the volumes at which production becomes viable.

The shorter flux paths through the iron and the air-gap reduce the fluxdensity in the back iron 7, i.e the iron below the slots 1, thus itbecomes possible to reduce the thickness of this iron and thus increasethe diameter of the motor shaft for a given motor size. The shaftdiameter thus becomes a larger proportion of the overall diameter of thesecondary side. This results in a stiffer shaft (less compliance) for agiven motor torque and thus pushes mechanical resonances out to higherfrequencies where they can be more easily filtered. The concentratedwinding 13 design with no pole shoes also allows the diameter of therotor or secondary side to be greater for a given overall motor diameterwhich results is a higher torque output.

FIG. 3 shows a linear motor side elevation in section with primary side2 and secondary side 4 whereby the primary side teeth 3 have curvedfaces 17 which results in a variable width air-gap 10 across the widthof the tooth 3. This can be more clearly seen in FIG. 5. The linearmachine does not require trapezoidal slots 1 as there is no curvature todeal with, and the inter-slot sections 15 and 16 remain parallel sidedand minimised in iron volume. FIG. 6 shows a further method ofgenerating a variable air-gap flux by using curved magnets 1 whichprovide a higher flux in the middle than at the edges due to theincreased thickness of magnetic material. This then creates in turn avariable air-gap flux across the width of the slot and thus the primaryside tooth. It would also be quite feasible to magnetise a rectangularmagnet such that the flux produced by the magnet was variable across itswidth in order to achieve the same result.

The design of the machine also results in levels of d and q axisinductance which can,be varied quite markedly with the level of primaryside coil current.

The concentrated winding structure 13 and the ratio of secondary side toprimary side poles 20 results in a rotating magnetic field in theair-gap 10 which contains a fundamental frequency and its harmonicswhereby the secondary side 4 is designed to interact only with the firstharmonic of the field to produce torque.

As the flux paths through the secondary side are optimised as alreadydescribed, it is possible to simplify the geometry of the secondary sideand to do without the complex cut-outs which are commonly used tophysically define the flux paths through the secondary side andthe-value of Lq, and which can also disturb the air-gap flux causingfurther unwanted harmonics of flux in the air-gap and hysteresis losses;it is thus possible to have a completely smooth secondary side surfaceand to have a substantially continuous iron path along the machinequadrature axis.

The width of a secondary side slot 29 is designed to be less than thewavelength of the first harmonic of the rotating electromagnetic fluxfield, preventing a closed flux path from forming within the dimensionsof a slot 1 and thus preventing circulating currents from flowing in themagnets within the slots 1. The dimensions 6, 7, 9, of the secondaryside 4 for a given motor are normally fixed, i.e. do not vary around oralong the motor, although there could be further optimisations whichrequire such modifications in order to further optimise the q and d axisinductances by modifying the d and q flux paths.

Figure References

-   1 Secondary side slot/magnet-   2 Primary side-   3 Primary side tooth-   4 Secondary side-   5 Secondary side inner surface-   6 h1 dimension-   7 h2 dimension-   8 Slot height or magnet thickness-   9 Thickness of secondary side iron-   10 Air-gap-   11 Secondary side base-   12-   13 Primary side winding-   14 Winding to phase connection-   15 Stabilising segment-   16 Inter-pole segment-   17 Curved tooth face-   18 Shaped secondary side slot and/or magnet-   19 Flat tooth face-   20 Secondary side pole-   21 Secondary side outer surface-   22 Tooth centre line-   23 Air-gap facing tooth side-   24 Slot centre line-   25 Slot air-gap surface-   26 Slot inner surface-   27 Machine axis-   28 Radius-   29 Slot width

1. Rotating A.C. synchronous interior permanent magnet machine withconcentrated windings whereby a radius (28) drawn from the axis of themachine (27) through the centre line (24) of the at least one interiorslot (1) would be substantially normal to the surface formed by theair-gap side of the slot (25) Characterised in that, each pole (20) onthe secondary-side (4) is provided with at least one interior axial slot(1), preferably open at least one axial-end, with dimensions h1 (6), h2(7) and d (9), with h1 (6) being the radial material thickness of thesecondary side between the air-gap surface, of the at least one slot(25) and the secondary side surface (21) measured along this radius(28), h2 (7) being the radial material thickness of the secondary sidebetween the inner surface (5) and the inner-lying wall (26) of the atleast one slot (1) measured along this radius (28) and d being theradial thickness of the secondary side, whereby the ratio h1/d lies inthe range 0.05-0.35
 2. Rotating A.C. machine according to claim 1characterised in that the ratio h2/d lies in the range 0.4-0.8. 3.Rotating A.C. synchronous interior permanent magnet machine withconcentrated windings whereby a radius (28) drawn from the axis of themachine (27) through the centre line (24) of the at least one interiorslot (1) would be substantially normal to the surface formed by theair-gap side of the slot (25) Characterised in that, each pole (20) onthe secondary-side (4) is provided with at least one interior axial slot(1), preferably open at least one axial end, with dimensions h1 (6), h2(7) and d (9), with h1 (6) being the radial material thickness of thesecondary side between the air-gap surface of the at least one slot (25)and the secondary side surface (21) measured along this radius (28), h2(7) being the radial material thickness of the secondary side betweenthe inner surface (5) and the inner-lying wall (26) of the at least oneslot (1) measured along this radius (28) and d being the radialthickness of the secondary side, whereby the ratio h1/d lies in apreferred range 0.18-0.23
 4. Rotating A.C. machine according to claim 3characterised in that the ratio h2/d lies in a preferred range 0.55-0.635. Rotating A.C. machine according to any of the claim 1 to 4,characterised in that the air-gap facing side (23) of each primary sidetooth (3) is shaped such that the combination of the tooth geometry andthat of the secondary side produces a variable width air-gap (10)circumferentially around the machine.
 6. Rotating A.C. machine accordingto any claim 5 characterised in that each primary side tooth (3) issubstantially cuboid, the air gap facing side (23) being thussubstantially flat generating a variable width air-gap (10) with thecurved surface (21) of the secondary side.
 7. Rotating A.C. machineaccording to claim 1 characterised in that the at least one slot (1) issubstantially trapezoidal in form.
 8. Rotating A. C. machine accordingto any of claims 1 to 4 characterised in that the arrangement of primaryside windings (13) and associated phase currents generate a rotatingelectromagnetic flux field at a fundamental frequency and at harmonicsof that frequency, the dominant frequency being the first harmonic. 9.Linear A.C. synchronous interior permanent machine with concentratedwindings and magnet slots (1) each slot (1) running substantiallyparallel to the air-gap facing surface of the machine secondary side(21) Characterised in that, each pole (20) on the secondary-side (4) isprovided with at least one interior slot (1) preferably open at leastone end whereby the material thickness (6) of the secondary sidemeasured between the secondary side air-gap surface (21) and the air-gapfacing surface of a slot (25) is denoted by h1, the material thickness(7) of the secondary side between its base surface (11) and theinner-lying surface (26) of a slot (1) is denoted by h2, the thicknessof the secondary side (9) is denoted by d, and the ratio h1/d is in therange 0.05-0.35.
 10. Linear A.C machine according to claim 8Characterised in that the ratio h2/d is in the range 0.4-0.8
 11. LinearA.C. synchronous interior permanent machine with concentrated windingsand magnet slots (1) each slot (1) running substantially parallel to theair-gap facing surface of the machine secondary side (21) Characterisedin that, each pole (20) on the secondary-side (4) is provided with atleast one interior slot (1) preferably open at least one end whereby thematerial thickness (6) of the secondary side measured between thesecondary side air-gap-surface (21) and the air-gap facing surface of aslot (25) is denoted by h1, the material thickness (7) of the secondaryside between its base surface (11) and the inner-lying surface (26) of aslot (1) is denoted by h2, the thickness of the secondary side (9) isdenoted by d, and the ratio h1/d is in preferred ranged 0.18-0.23 12.Linear A.C. machine according to claim 10 Characterised in that theratio h2/d is in a preferred range 0.55-0.63
 13. Linear A.C. machineaccording to one of claims claim 8 to 11 characterised in that thearrangement of primary side windings (13) and associated phase currentsgenerate a moving electromagnetic flux at a fundamental frequency, andat harmonics of that frequency, the dominant frequency being the firstharmonic.
 14. Linear A.C. machine according to one of claims 8 to 11characterised in that the geometry of at least one air-gap facing toothside (23) on the primary side (3) is such that the width of the air-gap(10) varies across that dimension of the tooth (3) parallel to thedirection of motion of the magnetic flux.
 15. Linear A.C. machineaccording to claim 12 characterised in that either: a) the at least onepermanent magnet (1) has a non-constant thickness (8) such that thestrength of the magnetic field due to the magnet varies across its width(29). b) the at least one permanent magnet (1) is magnetised such thatthe strength of the magnetic field due to the magnet varies across itswidth (29) c) a combination of a) and b)
 16. A.C. machine according toany of the claims 1 to 4 or 8 to 11 characterised in that the secondaryside surface (21) is substantially smooth.
 17. A.C. machine according toclaim 7 or 13 characterised in that each slot (1) contains only onesubstantially slot-sized permanent magnet whose width (29) is chosen tobe less than the wavelength of the first harmonic component of magneticflux.
 18. A.C. machine according to any one of the claims 1 to 4 or 8 to11 characterised in that the ferromagnetic material forming the physicalbarrier (16) between any two adjacent secondary side slots (1) in twoadjacent secondary side poles (20), is substantially parallel sided. 19.A.C. machine according to any one of the claims 1 to 4 or 8 to 11characterised in that the ferromagnetic material forming the physicalbarrier (15) between any two adjacent secondary side slots (1) withinone secondary side pole (20), has is substantially parallel sided. 20.A.C. machine according to claim 19 characterised in that the materialforming the barrier (15) remains in magnetic saturation under allspecified operating conditions of the motor.
 21. Method for producing asecondary side of a Concentrated winding, A.C. interior permanent magnetsynchronous machine according to any one of the claims 1 to 4 or 8 to 11characterised in that at least one pre-formed block of non-magnetisedpermanent magnet material is inserted into the at least one secondaryside slot (1) and the completed secondary side is then magnetised in asuitable magnetising fixture to produce the desired permanentmagnetisation.